If you do not know what they are, realms are servers on the phanaticmc network that you can setup how you want and join after connecting to phanaticmc. You just do the command /connect <servername> (the name can be whatever you want) for you and your friends to play on it

If you are the owner of the realm you can do the command /bluestart which gives you a link to a website with the control panel, you have access to the console, logs, enabling plugins, and all that good stuff for managing your server